Merriam-Webster's Learner's Dictionary
: difficult or painful experiences that people have in their lives or careers
▪ He has taken plenty of hard knocks in his life.
▪ The school of hard knocks [=the difficult experiences in his life] taught him how to be tough.
Do British English speakers use the phrase "school of hard knocks"?
I'm a Brit and that idiom is floating around in the back of my mind, not sure that I would ever actually use it, but have definitely seen/heard it. Don't know the origin of it though.